Augustine of Hippo
Catena Aurea · Matthäusevangelium 5
Ye have heard that it hath been said, Thou shalt love thy neighbour, and hate thine enemy. But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you and persecute you; That ye may be the children of your Father which is in heaven: for He maketh His sun to rise on the evil and on the good, and sendeth rain on the just and on the unjust. For if ye love them which love you, what reward have ye? do not even the Publicans the same? And if ye salute your brethren only, what do ye more than others? do not even the Publicans so? Be ye therefore perfect, even as your Father which is in heaven is perfect.
(Serm. in Mont. i. 21.) That there were degrees in the righteousness of the Pharisees which was under the old Law is seen herein, that many hated even those by whom they were loved. He therefore who loves his neighbour, has ascended one degree, though as yet he hate his enemy; which is expressed in that, and shalt hate his enemy; which is not to be understood as a command to the justified, but a concession to the weak.